1. Field of the Invention
This invention relates to the fastening of a plurality of members together and, more particularly, to a tube plug and method for plugging a heat exchanger tube in a nuclear steam generator.
2. Description of the Prior Art
In tube-type heat exchangers, a first fluid flows through the tubes of the heat exchanger while a second fluid surrounds the outside of the tubes such that heat exchange occurs between the two fluids. Occasionally, one of the tubes can become defective such that a leak either is impending or occurs therein which allows the two fluids to mingle. When this occurs, it is sometimes necessary to plug the tube so that the fluid does not flow through the tube thereby preventing leakage from the tube.
In nuclear reactor power plants, the tube-type heat exchangers are commonly referred to as steam generators. When a defect occurs in the tubes of a nuclear steam generator that allows the primary fluid in the tubes to mingle with the secondary fluid outside of the tubes, a more significant problem arises. Not only does this situation create an ineffective heat exchanger, but it also creates a radioactive contamination problem. Since the fluid flowing in the tubes of a nuclear steam generator may contain some radioactivity, it is important that it not be allowed to leak from the tubes and possibly contaminate the fluid surrounding the tubes and the equipment which it contacts. Therefore, when the possibility of a leak occurs in a nuclear steam generator heat exchange tube, the heat exchange tube is plugged so that the primary fluid is not permitted to flow through the tube.
There are several kinds of mechanical plugs that can be used to plug heat exchange tubes. One such device used to plug heat exchange tubes in nuclear steam generators is disclosed in U.S. Pat. No. 4,982,763 issued Jan. 8, 1991 to Klahn entitled "Plug Retainer". The plug retainer for retaining a tube plug comprises a locking cup having a bore therethrough and an externally threaded lower portion adapted to be threadably received in the open threaded end of the tube plug and a cap screw having a threaded shank threadably receivable in the threaded mandrel of the tube plug. The locking cup is crimped onto the cap screw by the use of flutes on the cap screw head. However, utilization of the locking cup requires the additional steps of threading the locking cup into the open threaded end of the tube plug and of crimping the locking cup into the flutes of the cap screw.
Another type of mechanical plug is disclosed in co-pending application Ser. No. 07/439,118 filed Nov. 20, 1989 by Haberman et al. and assigned to the assignee of the present invention. The tube plug for plugging a tube to prevent flow through the tube comprises a shell, an expander member, and a sealing member to seal the chamber defined by the shell. To lock or fasten the sealing member to the shell to prevent the sealing member and shell from separating, the sealing member and shell are welded together. The installation of the sealing member into the shell seals the open end of the shell to prevent coolant from entering the chamber of the shell and exerting pressure on the plug wall, and occupies space within the chamber of the shell to reduce the possibility of residual stresses within the plug wall. Although the tube plug successfully plugs tubes, welding the sealing member and the shell together requires the additional steps of welding and of inspection of the weld during the installation of the tube plug into the tube. Also, if the shell of the tube plug has been recessed a distance into the tube, the sealing member cannot be welded to the shell, and therefore the sealing member and shell cannot be locked or fastened together by welding.
Another type of mechanical plug is disclosed in co-pending application Ser. No. 07/699,406 filed concurrently herewith by Snyder et al. entitled "Tube Plug and Method for Plugging a Tube" and assigned to the assignee of the present invention. The tube plug for plugging a tube to prevent flow through the tube comprises a shell, an expander member, bolt means to seal the chamber defined by the shell, and locking means for securing the shell, bolt means and locking means together. The bolt means reduces the possibility of residual stresses within the plug wall. However, utilization of the locking means requires the additional steps of threading the locking means into the shell and of crimping the locking means into the bolt means.
Therefore, what is needed is a tube plug and method for installing a tube plug that automatically locks or fastens together the plurality of members comprising the tube plug by the installation of the bolt means into the shell of the tube plug.